PROJECT SUMMARY/ABSTRACT Distortion product otoacoustic emissions (DPOAEs) hold great promise as a means of providing a noninvasive window into the status of the cochlea that can both advance our scientific knowledge about cochlear function, and contribute a rapid test of hearing impairment to the clinical armamentarium. However, when measured at recommended primary-tone levels of about f1=65 and f2=55 dB SPL in patients with sensorineural hearing loss such as Veterans, poor signal-to-noise ratios (SNRs) often make clinical interpretations difficult. DPOAEs elicited by higher primary levels have better SNRs, but are both less sensitive and less frequency specific due to the associated basal spread of both excitation and DPOAE components. If these shortcomings could be overcome, the applicability of DPOAEs in the clinic would be greatly enhanced. The primary significance of the proposed program of research is to further our knowledge of ear-canal DPOAEs vs intracochlear DPs (iDPs) to improve DP-grams measured at moderate to high primary-tone levels for clinical purposes by: 1) understanding the basis of the apparent increase in frequency specificity of the -oct IT procedure by simultaneous measures of both the DPOAE and direct observations of the effects of the IT on f2 and iDP near the basilar membrane (BM) in a gerbil model; 2) comparing these intracochlear measures to those obtained with a noninvasive iDP technique where an f3 tone is paired with the iDP to produce a DPOAE used to infer the behavior of the iDP. This comparison will establish the validity of the noninvasive ear-canal method for examining the behavior of iDPs, and determine the differences between DPOAEs and iDPs; and 3) simultaneously measuring the response/growth or input/output functions for both the ear-canal DPOAE and the iDP using direct intracochlear pressure measures near the BM. With this approach, transfer functions for the DPOAE can be developed that predict the behavior of the iDP inside the cochlea. The planned studies will also compare the performance of the standard DP-gram procedure to two new procedures, the -oct IT DP- gram and the iDP-gram in a population of Veterans with noise-induced audiometric notches in the 4- to 6-kHz range. On the whole, the premise of the proposed experiments is that by determining the differences between conventional ear-canal DPOAEs and iDPs generated within the cochlea, a means towards improving the frequency specificity of clinical DP-gram tests obtained with moderate- to high-level primary tones will be provided. Accomplishing a more frequency-specific DP-gram will allow clinicians to reliably establish the pattern of outer hair cell damage using high-level primary tones to achieve adequate SNRs in Veterans with sensorineural hearing loss.